In recent times, disposal and processing of spent nuclear fuel are becoming an important global issue to be solved for nuclear power plants which are attracting attention as environmentally friendly energy. In Korea, the government has selected a pyrochemical process as a recycling technology of spent nuclear fuel and, in addition to development of related process techniques such as electrolytic reduction, electrolytic refining and electrolytic winning, research and development of molten-salt-related basic technology is being performed.
Molten salts are typically used as solvent for manufacturing aluminum, magnesium, sodium, and so on, metallurgical and electrochemical importance of the molten salts has been recognized from about 40 years ago, and research into molten salts is being actively performed. Molten salts may be generally classified into four kinds, chloride-based salts such as LiCl—KCl, LiCl—NaCl, KCl—LiCl—NaCl, and so on, fluoride-based salts such as LiF—NaF—KF, LiF—NaF, LiF, and so on, cyanide-based salts such as NaCN, KCN, and so on, and an oxygen-containing salts such as CaCl2—CaWO4, and so on. In addition, research into bromide-based and iodine-based salts is also being performed.
Such molten salts have the following advantages, which make them very beneficial as an electrolyte. First, these molten salts are electrochemically stable and have high overpotential for decomposition. In addition, these molten salts have high electrical conductivity, and refractor metal which is not electrolytically deposited in an aqueous solution can be electrolytically deposited. Further, flat and thick deposition is also possible.
The spent nuclear fuel is converted into a metal conversion body during the electrolytic reduction process, and moves to an electrolytic refining reactor formed of an anode basket, in which a metal conversion body is contained in a LiCl—KCl high temperature molten salt electrolyte, and a solid cathode, to collect uranium only. When a potential is applied to these electrodes, uranium, rare earth metal and trans uranium (TRU) elements are oxidized and melted at a surface of the anode, and only uranium ions are selectively moved to the cathode to be deposited thereon as a pure uranium metal.
The electrolytic winning process of the pyrochemical process, which is a follow-up process of the electrolytic refining process, is recognized as a very important process in order to perform nuclear non-proliferation, because both uranium and TRU are simultaneously collected using a liquid cadmium cathode (LCC). In operation of the electrolytic refining and winning processes, precise measurement of concentration of uranium existing in the molten salts is very important.
While the conventional art collects molten salts and measures concentration of the molten salts using analysis techniques such as ICP-AES or ICP-MS, real time on-site analysis of uranium is essentially needed to smoothly perform the processes.
The uranium exists as chloride in the molten salts, and mainly has a type of uranium trichloride (UCl3) with an oxidation number of 3+. Since the uranium trichloride molten salts have a thick red wine color whereas uranium tetrachloride (UCl4) with an oxidation number of 4+ has a thin yellowish green color, it will be appreciated that a strong light absorption peak is provided in the visible light region.
Ultraviolet-visible light absorption spectrometry, which is generally used, is a method of quantitatively or qualitatively analyzing concentrations of elements in a sample using the degree to which the sample absorbs ultraviolet or visible light. Light absorbance of a molecule at the ultraviolet and visible light region is relevant to an electron structure of the molecule. A spectrum cell in which a sample is inserted generally uses a quartz cell at an ultraviolet region. When only a peak is observed at the visible light region, a general glass cell may be used. In directly applying absorption spectrometry to the pyrochemical process, in order to employ conventional absorption spectrometry, a quartz window through which ultraviolet-visible light can pass must be provided in a process apparatus. However, corrosion may occur due to a process material such as Li2O, and so on, included in the molten salts, affecting the integrity thereof.
The quantitative analysis using the absorption spectrometry follows Beer's Law (A=ebC), i.e., intensity of light (light absorbance, A) absorbed by a sample is in proportion to concentration (C) of the sample. Here, b represents a width of a spectrum cell in which a sample is contained. When the concentration is too thick, due to a phenomenon in which an absorption peak is saturated, the absorption saturation can be solved by reducing the width of the cell. Accordingly, in order to perform on-site real time analysis of the process material, the width of the spectrum cell must be adjusted according to a concentration range of uranium existing in the molten salts. In addition, since the quartz is likely to be broken due to corrosion by lithium oxide in the molten salts during a long period of operation, the quartz window must be periodically exchanged with a new one.